The present invention relates to an air drying system to be used in conjunction with an onboard oxygen generating system (OBOGS) of an aircraft, such as a military fighter plane. More particularly, the present invention relates to an air drying system having a hollow fiber membrane filter for use with a pressure swing adsorption (PSA) OBOGS system.
The air drying system takes compressed air to generate oxygen enriched air (OEA), with nitrogen enriched air (NEA) being generated as a waste gas. The source of compressed air can be bleed air from the aircraft engine or auxiliary power unit (APU), or can be from ambient or aircraft cabin air that is pressurized using a feed air compressor. In all cases, the compressed air contains a certain amount of water vapor/humidity. In one embodiment of the invention, prior to gas separation via the PSA molecular sieves, the air passes through a hollow fiber membrane (HFM) filter to extract the water vapor from the air. The HFM filter is then swept by either pre-PSA dried air or post-PSA NEA to dispel any collected water vapor to atmosphere thereby allowing for continual water extraction within the HFM filter unit.
To operate effectively, OBOGS units require a certain amount of pressure at the air inlet. Important to the present invention, the pressure drop across the HFM filter, along with the purge flow used to sweep the HFM filter, necessitates that a higher inlet air pressure be provided to the system. However, an aircraft, such as a fighter jet, may be subject to instances of low inlet pressures to the OBOGS unit. For instance, bleed air pressure from the engines may be low when the engines are at a low throttle setting. Fighter pilots may also incur instances of increased oxygen demand, such as when flying at high altitudes or when performing maneuvers which generate significant G-forces. In these low inlet pressure/high oxygen demand situations, an embodiment of the air drying system of the present invention includes a bypass valve to circumvent the HFM filter unit such that any air passing through the inlet passes directly through the molecular sieves of the PSA unit thereby maximizing production of OEA for delivery to the pilot. Any water adsorbed by the molecular sieve of the PSA unit will be subsequently desorbed by HFM filter dried air from the HFM filter once the low inlet pressure/high oxygen demand condition has been resolved and the inlet air first passes through the HFM filter of the dryer unit.